A series of film holders designed for cut strip negatives. Other designs use an 'S-bend' slot to keep the negative flat, but that assumes you've left the entire roll of film uncut. If you want to scan the frame at the end of a cut strip, they're not much use.
This design uses a shaped base and a weighted top piece that fits into it to press the film down flat. It only holds the film around the edges of the frame, so there shouldn't be anything touching the adjacent frames.
The weight is provided by short lengths of 8mm square steel bar, glued into a recess in the top part of the film holder. I painted them black so they don't go rusty.
The base is printed the right way up, and the top is printed upside down so the critical dimensions are more accurate. That means you'll need to switch on the supports so it prints them inside the recess for the weights. A raft will also help.